Extended X-ray Absorption Fine Structure Spectroscopy Analysis Research Articles

Molecular dynamics simulation and X-ray absorption spectroscopy to characterize the local structure and dynamics of calcium ion in sulfonated polystyrene

Molecular Dynamics (MD) Simulation and the Extended X-ray Absorption Fine Structure spectroscopy (EXAFS) were combined to characterize the “averaged” coordination structures and short-time dynamics of the calcium-doped sulfonated polystyrene (CaSPS) at the atomistic level. MD simulation of the CaSPS ionomer implies that there is evidence for the Ca...O coordination from both the sulfonate group and oxygen atoms. The EXAFS fitting procedure was compared between the conventional technique using the atomic coordinates from the standard CaO crystal and the method using selected snapshots from MD trajectories. The averaged Ca…O distance and the coordination number in the first solvation shell from the simulation are in good agreement with the EXAFS spectra (2.5, 2.4 Å and CN = 5.8, 5.7 from MD, EXAFS analysis, respectively). From MD simulation, the cases of 5–7 oxygen atoms from 4 to 5 sulfonated groups coordinating with calcium are the most frequently seen. The best fit of EXAFS data using the atomic coordinates from MD snapshot suggests that there is only one dominant shell for Ca…O coordination which is six oxygen atoms from five different sulfonated groups and the other one from a water molecule. The interaction between Ca2+ ion and sulfonate group strongly slows down the dynamics of these species in the cooperative manner whereas water molecules exhibit much faster dynamics.

Identification of structural phases in ferroelectric hafnium zirconium oxide by density-functional-theory-assisted EXAFS analysis

Crystalline phase identification for hafnium-based ferroelectrics by diffraction techniques has been elusive. We use density-functional-theory (DFT)-assisted extended X-ray absorption fine-structure spectroscopy (EXAFS) to determine the crystal symmetry of thin hafnium zirconium oxide (Hf0.46Zr0.54O2) films grown by atomic layer deposition. Ferroelectric switching in TiN/Hf0.46Zr0.54O2/TiN metal–insulator–metal capacitors is verified. Grazing-incidence fluorescence-yield mode Hf L3 and Zr K absorption edge EXAFS data are compared with reference data calculated from DFT-based atomic coordinates for various structural phases of Hf0.5Zr0.5O2. Via EXAFS multiphase fitting, we confirm that the frequently invoked polar orthorhombic Pca21 phase is present in ferroelectric hafnium zirconium oxide, along with an equal amount of the nonpolar monoclinic P21/c phase. For comparison, we verify that paraelectric HfO2 films exhibit the P21/c phase.

Cd2+ incorporation in small-pore LEV/ERI intergrown zeolites: A multi-methodological study

Small-pore zeolites are successfully employed as catalysts, sorbents and molecular sieves. Their physiochemical properties can be tuned by modifying their extraframework cation (EF) composition via ion exchange. In this study, we investigate the crystal structure of a Cd-exchanged levyne ( LEV ) intergrown with erionite ( ERI ) by combining Single Crystal X-ray Diffraction (SCXRD), Molecular Dynamic simulations (MD) and Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). Data obtained from the different techniques consistently indicate that Cd 2+ in LEV is arranged in a nearly ordered fashion. In contrast, strong disorder of the EF species (Cd 2+ and H 2 O) is observed in the ERI cavities. Here, Cd 2+ forms aqueous complexes that are more mobile in comparison to LEV , where Cd 2+ bonds to both H 2 O and framework-oxygen atoms. The formation of Cd-clusters is excluded based on EXAFS analysis. Finally, to discriminate between thermal and static disorder, we propose a new approach based on combined MD and geometry optimization. • The crystal structure of Cd-LEV and Cd-ERI was determined by SCXRD. • Cd 2+ has a fairly regular octahedral coordination in Cd-LEV. • Cd 2+ is strongly disordered in the eri cavity. • Cd aqueous complexes form in Cd-ERI. • The disorder of the EF species was resolved by combining MD simulations and XAFS.

Adsorption of antimony onto iron oxyhydroxides: Adsorption behavior and surface structure

Antimony is detected in soil and water with elevated concentration due to a variety of industrial applications and mining activities. Though antimony is classified as a pollutant of priority interest by the United States Environmental Protection Agency (USEPA) and Europe Union (EU), very little is known about its environmental behavior and adsorption mechanism. In this study, the adsorption behaviors and surface structure of antimony (III/V) on iron oxides were investigated using batch adsorption techniques, surface complexation modeling (SCM), X-ray photon spectroscopy (XPS) and extended X-ray absorption fine structure spectroscopy (EXAFS). The adsorption isotherms and edges indicated that the affinity of Sb(V) and Sb(III) toward the iron oxides depended on the Sb species, solution pH, and the characteristics of iron oxides. Sb(V) adsorption was favored at acidic pH and decreased dramatically with increasing pH, while Sb(III) adsorption was constant over a broad pH range. When pH is higher than 7, Sb(III) adsorption by goethite and hydrous ferric oxide (HFO) was greater than Sb(V). EXAFS analysis indicated that the majority of Sb(III), either adsorbed onto HFO or co-precipitated by FeCl3, was oxidized into Sb(V) probably due to the involvement of O2 in the long duration of sample preservation. Only one Sb–Fe subshell was filtered in the EXAFS spectra of antimony adsorption onto HFO, with the coordination number of 1.0–1.9 attributed to bidentate mononuclear edge-sharing (2E) between Sb and HFO.

Formation of Crystalline Zn–Al Layered Double Hydroxide Precipitates on γ-Alumina: The Role of Mineral Dissolution

To better understand the sequestration of toxic metals such as nickel (Ni), zinc (Zn), and cobalt (Co) as layered double hydroxide (LDH) phases in soils, we systematically examined the presence of Al and the role of mineral dissolution during Zn sorption/precipitation on γ-Al(2)O(3) (γ-alumina) at pH 7.5 using extended X-ray absorption fine structure spectroscopy (EXAFS), high-resolution transmission electron microscopy (HR-TEM), synchrotron-radiation powder X-ray diffraction (SR-XRD), and (27)Al solid-state NMR. The EXAFS analysis indicates the formation of Zn-Al LDH precipitates at Zn concentration ≥0.4 mM, and both HR-TEM and SR-XRD reveal that these precipitates are crystalline. These precipitates yield a small shoulder at δ(Al-27) = +12.5 ppm in the (27)Al solid-state NMR spectra, consistent with the mixed octahedral Al/Zn chemical environment in typical Zn-Al LDHs. The NMR analysis provides direct evidence for the existence of Al in the precipitates and the migration from the dissolution of γ-alumina substrate. To further address this issue, we compared the Zn sorption mechanism on a series of Al (hydr)oxides with similar chemical composition but differing dissolubility using EXAFS and TEM. These results suggest that, under the same experimental conditions, Zn-Al LDH precipitates formed on γ-alumina and corundum but not on less soluble minerals such as bayerite, boehmite, and gibbsite, which point outs that substrate mineral surface dissolution plays an important role in the formation of Zn-Al LDH precipitates.

Local structural phase determination of Ni silicide thin films using EXAFS

AbstractThe local structural characterization of novel contact materials (nickel silicides) on N2+ implanted Si(100) substrates have been performed using Extended X‐ray‐Absorption Fine‐Structure Spectroscopy (EXAFS). The crystal phases of nickel silicides that are stabilized on Si (100) substrates (NiSi or NiSi2) upon varying implantation and post deposition annealing conditions were identified. The questions addressed are whether the final stabilized crystal phases of the nickel silicide layers could be identified and the fraction of the various crystal symmetries could be determined. The identification of the stabilized crystal phases in the nickel silicide formation process is crucial, as it was shown that a Si rich Ni silicide layer at the NiSix/Si interface causes a significant reduction in the electron Schottky barrier height (SBH). EXAFS analyses using experimental and theoretical references were performed on nickel silicide thin films on silicon substrates exposed to different N2+ ion doses prior to nickel deposition and silicidation processes. Using EXAFS analyses, the stabilized phases (e.g. NiSi vs. NiSi2, etc.) and their ratios were determined. EXAFS was proved to be sensitive to the subtle modifications of the crystal structures introduced by these modifications in the processing conditions. EXAFS analyses of the experimental reference structures (NiSi, NiSi2 powder samples) clearly exhibited the robust differences in the Fourier Transformed (FT) EXAFS data in the first shell region. EXAFS fits using theoretical structures corresponding to NiSi and NiSi2 symmetries have successfully identified the distinct crystal phases present in nickel silicide thin films subjected to different doses of N2+ ion implantation. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectroscopic Characterization of Cobalt-Containing Mesoporous Materials

Cobalt-containing mesoporous materials that have been prepared using different procedures have been comparatively characterized by transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM/EDS), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near edge spectroscopy (XANES), and ultraviolet-visible (UV-vis), near-infrared (NIR), and mid-infrared (mid-IR) spectroscopies, and the results provide new insights into the local environment and properties of cobalt in this type of material. TEM/EDS analyses have shown that tetraethyl orthosilicate (TEOS) may be less appropriate as a silicon source during the syntheses of cobalt-containing mesoporous materials, because the distribution of cobalt throughout the framework may become uneven. EXAFS has been determined to be the most suitable method for direct verification of framework incorporation, by identifying silicon as the backscatterer in the second shell. Such a direct verification may not be obtained using UV-vis spectroscopy. From EXAFS analyses, it is also possible to distinguish between surface-bound and framework-incorporated cobalt. There is a good agreement between the results obtained from XANES and UV-vis regarding the coordination symmetry of cobalt in the samples. The presence of cobalt in the silica framework has been determined to create Lewis acid sites, and these acid sites are suggested to be located at tetrahedral cobalt sites at the surface.

RESIDENCE TIME EFFECTS ON ARSENATE SURFACE SPECIATION AT THE ALUMINUM OXIDE-WATER INTERFACE

Bioavailability of metals/metalloids is often rate limited by contact time (i.e., residence time) in soils and sediments, resulting in irreversible reactions. The fate and transport of the contaminants must be predicted/modeled not only on short-term (<48 h) adsorption/desorption studies but also on long-term (months-years) reactions. However, there is very little information on the long-term effects of metal/metalloid partitioning reactions in soils and soil components. In this study, residence time effects (3 days–1 yr) on As(V) adsorption/desorption reactions and on As(V) surface speciation at the aluminum oxide-water interface were investigated using batch adsorption/desorption experiments coupled with time-resolved Extended X-ray Absorption Fine Structure spectroscopy (EXAFS). Biphasic As(V) adsorption kinetics were observed at pH 4.5 and 7.8, and whereas the reaction at pH 4.5 was nearly completed after 3 days, slow adsorption continued at pH 7.8 after 1 year. The longer the residence time (3 days–1 yr), the greater the decrease in As(V) desorption at both pHs, suggesting nonsingular reactions. EXAFS analyses on aged As(V) reacted aluminum oxide at both pHs showed that As-Al interatomic distances were 3.11 − 3.14 Å (±0.13 Å) in all of the aged samples (3 days to 1 yr) at pH 4.5 and 7.8, suggesting that predominantly bidentate binuclear bonding environments were present. As a point of interest, X-ray Absorption Near Edge Structure spectroscopy (XANES) features suggested some changes in the local chemical structure of adsorbed As(V) with aging. The surface transformations such as (i) a rearrangement of surface complexes and/or (ii) a conversion of surface complexes into aluminum arsenate-like precipitates might be important chemical factors responsible for the decrease in As(V) reversibility with aging.

Hydration of the calcium ion. An EXAFS, large-angle x-ray scattering, and molecular dynamics simulation study.

The structure of the hydrated calcium(II) ion in aqueous solution has been studied by means of extended X-ray absorption fine structure spectroscopy (EXAFS), large-angle X-ray scattering (LAXS), and molecular dynamics (MD) methods. The EXAFS data displayed a broad and asymmetric distribution of the Ca-O bond distances with the centroid at 2.46(2) A. LAXS studies on four aqueous calcium halide solutions (1.5-2 mol dm(-)(3)) gave a mean Ca-O bond distance of 2.46(1) A. This is consistent with a hydration number of 8 determined from correlations between mean distances and coordination numbers from crystal structures. The LAXS studies showed a second coordination sphere with a mean Ca.O(II) distance of 4.58(5) A, and for the hydrated halide ions the distances Cl.O 3.25(1) A, Br.O 3.36(1) A, and I.O 3.61(1) A were obtained. Molecular dynamics simulations of CaCl(2)(aq) were performed using three different Ca(2+)-OH(2) pair potentials. The potential from the GROMOS program gave results in agreement with experiments, i.e., a coordination number of 8 and an average Ca-O distance of 2.46 A, and was used for further comparisons. Theoretical EXAFS oscillations were computed for individual MD snapshots and showed very large variations, though the simulated average spectrum from 2000 snapshots gave satisfactory agreement with the experimental EXAFS spectra. The effect of thermal motions of the coordinated atoms is inherent in the MD simulation method. Thermal disorder parameters evaluated from simulated spatial atom distribution functions of the oxygen atoms coordinated to the calcium ion were in close agreement with those from the current LAXS and EXAFS analyses. The combined results are consistent with a root-mean-square displacement from the mean Ca-O distance of 0.09(2) A in aqueous solution at 300 K.

In Situ Study of the Bonding of Impurities in Metal-Impurity-Metal Laminate Composites

The local structure of interface or near interface impurity elements has been observed by means of extended x-ray absorption fine structure spectroscopy (EXAFS). Multilayered samples were prepared using reactive ion sputtering, electron beam evaporation , and molecular beam deposition methods with vacuum conditions ranging from 10−5 to 10−9 Pa. The interface layers of titanium, gallium, and arsenic were deposited in partial to multiple atom layers alternating with 5 to 10 nm thick matrix layers of nickel, cobalt, or aluminum. Oxide phases, atoms in solution, and local ordering were identified. Ti oxides and silicon arsenide were used as standards for the EXAFS analysis. Auger electron spectroscopy complemented the EXAFS analysis.